Methods and devices are known for the continuous preparation of coated products with reduced dust emission from aggregate and binder which are introduced into a drum which plays the part of both dryer and coater. This drum, which is of cylindrical or frusto-conical shape, is rotated about its longitudinal axis and receives continuously at one of its ends aggregate or a mixture of aggregate and powdery matter or a mixture of aggregate with or without the addition of powdery matter, and of binder.
In the case where the drum is supplied with aggregate this is generally cold and moist and in certain cases re-moistening of the aggregate is carried out before its entry into the drum. In these known methods the addition of binder is effected either at the inlet to the dryer and coater drum or in a zone of the drum close to the inlet and bringing about mixing of the aggregate with the binder, while the aggregate still contains all its moisture content and is at a rather low temperature. Similarly in the case where the mixture of aggregate with or without the addition of powdery matter and of binder is effected prior to the introduction of the aggregate into the drum, binder, for example, molten bitumen, comes into contact with a material which is cold and moist.
This manner of operating in which the addition of binder takes place prior to any drying of the aggregate has the advantage of binding the fine particles of aggregate before they have been able to be placed in suspension in the gases in circulation in the drum, which accordingly reduces the emission of dust into the outlet gases from the dryer and coater drum.
This manner of operating has, however, the disadvantage of putting the binder which is at high temperature in contact with an aggregate at ambient temperature or at a slightly higher temperature. That causes coagulation of the binder on the granules of aggregate which is detrimental to good distribution of the binder. The aggregate-binder area of contact is not a maximum and there is emission of dust by the particles not covered. the efficacy of this technique as to the reduction of the emission of dust and to the quality of the coated matter is therefore limited.
On the other hand, the addition of the binder into a zone adjoining the inlet to the drum has the disadvantage of introduction of the binder into a zone adjoining the flame of the burner which is generally introduced through the inlet end of the drum, the binder being also exposed to radiation from, and to contact with, the flame and gases at very high temperature, which creates a risk of causing alteration of its properties. On the other hand because the aggregate is kept cold and moist before the addition of the binder, it has not been able to release water vapor, the presence of which in the mixing zone might play a moderating part in the oxidizing effect of the burner.
For all these reasons the methods of coating in a dryer-coater drum with reduced dust emission do not operate under optimum conditions for good spreading of the binder over the particles of the aggregate.
Methods and devices for coating are also known, in which the introduction of the binder is effected in a zone remote from the inlet of the aggregate to the drum and in which the circulation of the hot gases is effected in conterflow to the circulation of the aggregate, the burner being arranged at the outlet end of the dryer-coater drum. These methods assume, however, the employment of a water-binder mixture with a relatively high water content, which is not favorable to the efficiency of the operation since this water must be eliminated by evaporation in the dryer-coater drum.
Finally the methods and devices at present known do not achieve good incorporation of powdery matter with the aggregate before the introduction of the binder, which creates a risk of causing the formation of conglomerates of powdery matter and binder, resulting in a lack of homogeneity in the final product. The powdery matter constituted by particles smaller than approximately 80 .mu. is necessary in quite definite proportions in coated products in order to impart sufficient cohesion to these coated products, the properties of the highway surfacing achieved with coated products being optimum for a certain content of powdery matter. The standards in force in certain countries moreover provide for the contents of fine-grain materials. Thus to aggregates which may already contain a certain proportion of powdery matter one may be led to add fine sand, cement or lime, for example. The addition of powdery matter is effected generally on the means of bringing the aggregate to the drum, which introduces into this drum non-homogenized masses of aggregate and powdery matter.